1. Field of the Invention
The invention concerns a method to determine the value of a tube voltage of an x-ray tube of an x-ray device in order to acquire x-ray projections in a multiphase examination of a defined tissue to be examined of a patient, in order to generate at least one image of the defined tissue to be examined in each phase of the multiphase examination. The invention also concerns an x-ray device (in particular a computed tomography apparatus) which is designed to execute such a method. The invention also concerns a non-transitory, computer-readable data storage medium encoded with programming instructions that enable a processor or computer, in which the storage medium is loaded, to execute a method of the type described above.
2. Description of the Prior Art
In medical technology, given imaging with x-ray radiation, for example in computed tomography (CT), it is always sought to apply an optimally low dose of x-ray radiation to a patient to generate one or more images of a tissue of the patient. However, a defined image quality must also be achieved in order to be able to answer the clinical question at issue using the generated image or images, which requires a minimum dose. Relevant measures of image quality are the image noise or the image contrast, for example. In particular, the image contrast of defined elements and materials (such as iodine used as a contrast agent in computed tomography) has a relatively strong dependency on the spectrum of the x-ray radiation that is used. Given the use of relatively low tube voltages, the spectrum of the x-ray radiation is such that the image contrast of iodine increases. Inasmuch, given the use of iodine the dose of x-ray radiation that is applied to a patient to achieve an optimally high image quality also depends on the spectrum of the x-ray radiation.
In CT angiograms to show blood-carrying vessels—in which the visibility of iodine is practically of exclusive importance—the dose of x-ray radiation applied to a patient is therefore reduced by the use of relatively low tube voltages (see also M. J. Siegel et al., “Radiation Dose and Image Quality in Pediatric CT: Effect of Technical Factors and Phantom Size and Shape”, Radiology 2004; 233: Page 515 to 522).
In “Automatic selection of tube potential for radiation dose reduction in CT: A general strategy”, L. Yu et al. propose a method to select a tube voltage suitable for a specific examination with regard to a reduction of the dose of x-ray radiation that is to be applied to a patient, in which method an iodine contrast-to-noise ratio is used as an image quality index in connection with a noise constraint parameter a in order to be able to characterize and account for the different requirements for the image quality for different examinations. Different noise constraint parameters α are available for adaptation for different examinations. With the noise constraint parameter it should be achieved that the absolute image noise does not exceed a certain value. For a CT angiogram in which the iodine contrast-to-noise ratio is practically of exclusive relevance, the noise constraint parameter α is selected between 1.5 and 2.0. The noise constraint parameter α is selected between 1.1 and 1.25 for breast, torso or pelvic examinations with contrast agent, and α is selected equal to one for breast, torso or pelvic examinations without contrast agent. The tube voltage with which the lowest dose of x-ray radiation is applied to the patient for a specific examination is determined based on a “relative dose factor” (RDF) into which the contrast-to-noise ratio of iodine and the noise constraint parameter α enter.
A particular problem results in a multiphase examination of a specific tissue—the liver, for example—of which at least one image is respectively generated in different phases or in different states in order to be able to differentiate different types of lesions in the liver from one another for diagnostic purposes. In the case of the liver, the different phases or states are produced by the administration of contrast agent which is taken up at different temporal rates by the lesions of different types. The multiphase examination of the liver therefore includes what is known as a native scan of the liver without contrast agent as a first phase of the examination, a scan of the liver after the administration of a contrast agent in a second arterial phase of the liver, and a scan of the liver after the administration of a contrast agent in a third venous phase of the liver, in which scans respective images of the liver are generated. In order to be able to assess the time variation of the absorption of contrast agent by the lesions of different types, it is necessary for a significant acquisition parameter of the x-ray device (such as the tube voltage) to coincide or be the same in all scans, i.e. in all acquisitions of x-ray projections. However, this requirement is generally not compatible with the fact that the minimum dose of x-ray radiation should be applied to the patient in each scan while maintaining a high image quality. This is due to the fact that the liver is different during the native scan and the two scans with contrast agent whose concentration is different in the arterial phase and venous phase of the liver, and the fact that the image quality required by the user can be different in the different phases. Therefore different optimal tube voltages for the three phases can result with regard to the lowest dose of x-ray radiation to be applied to the patient in each scan, in particular with the necessary adherence to or compliance with the technical limits of the x-ray device with regard to the ability to adjust acquisition parameters (such as the tube voltage, the tube current, etc.).